Heat transfer surface internal coils

For low to moderate heat duties (in terms of heat duty per unit of vessel volume), a vessel jacket is usually adequate to provide the required heat transfer surface. As heat duty increases, internal heat transfer surfaces (helical coils. 

Commonly used heat-transfer surfaces are internal coils and external jackets. Coils are particularly suitable for low viscosity Hquids in combination with turbine impellers, but are unsuitable with process Hquids that foul. Jackets are more effective when using close-clearance impellers for high viscosity fluids. For jacketed vessels, wall baffles should be used with turbines if the fluid viscosity is less than 5 Pa-s (50 P). For vessels equipped with cods, wall baffles should be used if the clear space between turns is at least twice the outside diameter of the cod tubing and the fluid viscosity is less than 1 Pa-s (10... 

Fluidized bed reactors typrcally are vertical cylindrical vessels equipped with a support grid and feed sparger system for adequate fluidization and feed distribution, internal cooling coils for heat removal, and either external or internal cyclones to minimize catalyst carryover. Fluidizauon of the catalyst assures intimate contact between feed and product vapors, catalyst, and heat-transfer surfaces, and results in a uniform temperature within the reactor. Reaction heat can be removed by generating steam within the cooling coils or by some oilier heat-transfer medium. 

Equipment Considerations When the solute has a large heat of solution and the feed gas contains a high concentration of solute, as in absorption of HCl in water, the effects of heat release during absorption may be so pronounced that the installation of heat-transfer surface to remove the heat of absorption may be as important as providing sufficient interfacial area for the mass-transfer process itself. The added heat-transfer area may consist of internal cooling coils on the trays, or the liquid may be withdrawn from the tower, cooled in an external heat exchanger, and then returned to the tower. 

When the heat transfer is through internal coils or tubular baffles, the difference between the inner and outer heat transfer surfaces may be significant. 

Use internal pipes or coils. For laminar conditions, consider using a hollow agitator for additional heat transfer surface. 

Jackets, internal helical pipe coils, tube baffles, and plate coil baffles are used to provide heat-transfer surface area. The only surface area effective for heat transfer is that portion that is wetted by both service and process fluids. The effective heat-transfer area for some items may be determined as follows ... 

Most industrial fermentors incorporate heat-transfer surfaces, which include (i) an external jacket or external coil (ii) an internal coil immersed in the liquid and (iii) an external heat exchanger, through which the liquid is recirculated by a pump. With small-scale fermentors, approach (i) is common, whereas approach (iii) is sometimes used with large-scale fermentors. These heat transfer surfaces are used for... 

Heat transfer is a relevant feature of bioreactor operation, although biological reactions do not display high exo- or endothermicity typical of many chemical reactions, nor do they occur at such high temperatures. Nevertheless, the temperatures at which enzyme- or cell-catalyzed reactions (including fermentations) occur have to be controlled under strict limits. Hence, bioreactors incorporate heat transfer surfaces either as external jackets or coils, as internal coils, or as external heat exchangers. These surfaces are used to adjust the temperature of the stream... 

Heat Transfer Surfaces. When the process requires heat addition to or removal from the process fluid, the mixing tank must be equipped with appropriate heat transfer surfaces. Liquid motion supplied by the mixer enhances the heat transfer coefficient. Commonly used heat transfer surfaces, shown in Figure 6-8, include jackets, internal helical coils, and internal baffle coils. A jacket can be a tank outside the main tank, baffled, half-pipe, or dimpled. Each of these heat transfer surfaces can also be used in combination with a single coil or multiple heating coils installed within the space between the impeller and the tank wall. A suitable heat transfer fluid must be supplied on the service side of the heat transfer surfaces. 

The temperature in stirred tank reactors may be influenced by chemical or physical reactions within the tank. Cooling or heating devices might be required to control the process temperature. In many endothermic processes heat has to be added to raise and maintain the temperature of the bulk. In other exothermic processes heat is removed to avoid hot spots. Heating and cooling of the process fluid are accomplished by heat transfer between the process fluid and a heating or cooling media that is circulated within a closed heat transfer surface. Different types of heat transfer equipment are used in industrial processes such as jackets, external or internal helical coils, as sketched in Fig. 7.10. Heat transfer from the bulk of the tank to the heat transfer medium can be calculated by the standard heat transfer model ... 

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

Dg = Mean or centerline diameter of internal coil helix, mm (ft) hj = heat transfer coefficient on inside surface of jacket = viscosity at bulk fluid temperature, [(N s)/m ][kg/(m sec)] = viscosity at die wall temperature, [(N s)/m ][kg/(m sec)]... 

If the rate of heat transfer to or from the broth is important, then the heat transfer area per unit volume of broth should be considered. As the surface area and the liquid volume will vary in proportion to the square and cube of the representative length of vessels, respectively, the heat transfer area of jacketed vessels may become insufficient with larger vessels. Thus, the use of internal coils, or perhaps an external heat exchanger, may become necessary with larger fermentors. 

A jacketed vessel, as shown in Figure 3.23, is often used for maintaining temperature however, it has limited surface area and low heat transfer coefficients. Sometimes internal reactor cooling coils are also used to provide additional heat transfer area. In order to manipulate the heat transfer, maximum flow is maintained in a circulation loop, while the jacket temperature is adjusted by bringing in and letting out coolant. The reactor temperature controller provides a setpoint to the jacket temperature controller. Heat transfer is linear and proportional to the temperature difference. 

Internal coils can also be used to increase heat transfer by supplying additional surface area. However, the coil surfaces may be subject to encrustation and may be difficult to clean. 

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